<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <script>
        var value = window.prompt("输入你的年龄","xxx");
        //括号内后一个参数是默认值
        if(value>18){
            alert("能进网吧");
        }
    </script>
</body>
</html>